Shield recall

[BuckeyeChuck]

Rastoff provided material from a guy who could actually do the (relatively simple) math for the position-velocity-acceleration expressions that are first-quarter material for any physics student whose course is based in differential math.

Some of the physics pontification up to this contribution was laughable for the obvious flaw in all of it: failure to identify that the acceleration in question occurs when the pistol hits something solid, not when it is dropped or thrown. Obviously your arm cannot exert 25g on a pistol to throw it, and obviously it won't experience 25g while accelerating in freefall. And just as obviously, these observations while correct are completely irrelevant.

It's apparent what's happened here: some posters who actually understand the simple physics involved assumed that everybody understood that the inertia safety is designed to prevent accidental discharge *when the gun stops*. Apparently, this is a bad assumption so a straightforward statement may clear up the issue:

The M&P Shield's inertia safety is present to prevent the trigger from discharging the gun by experiencing sufficient force due to deceleration if the gun hits something and decelerates rapidly.

The assertion that a pistol trigger can't experience 25g of acceleration when the gun collides with a hard surface after being dropped even from a height of one meter is laugh-out-loud ridiculous. This strikes me as the guy at the local car show who is sure that if he just keeps adding gears to his car, he could drive 250 MPH. Or the guy who knows that if you just put wind turbines all around a car, you could capture all the energy of the car's motion and put it back into the car so that it wouldn't burn any gas once it's up to speed. Or (and this one is one of my favorites) the guy who positively knows that covering an airliner with solar cells would greatly reduce the amount of fuel consumed by the airliner.

Thankfully somebody making some basic assumptions showed just how ridiculous is the notion that the trigger can't ever see that kind of acceleration.

I'll be checking my new-to-me Shield when I get home. Despite the errant pontifications in this thread, I want to be sure my gun will not fire when dropped because the physics say it can happen.

 

[Rastoff]

Thank you for that BuckeyeChuck.

It is never my intention to put someone down, though it may seem that way at times. There is a reason there are so few physicists in this world; physics isn't simple. The formulas are deceptively simple looking. Alas, while E=mc^2 looks simple, the logic, reasoning and application of it is not.

I tend to get trapped in some of these discussions. Like the fact that Centrifugal force is imaginary and doesn't exist. There are those who will argue until they pass out that it's real because they can feel it. When in fact, what they are really feeling is Centripetal force which is the force pushing in rather than out.

I understand why we think this way. As we turn in a corner we feel like something is pushing out so, it makes sense that centrifugal force is what we believe. Unfortunately, the math doesn't support it.

It's the same with things like G forces. It's difficult to accept that something as simple as dropping a gun can create numbers as high as 1,500g's; even if you see the math. Those numbers seem gigantic if you're not used to working with them.

My goal, as always, is education. We all become better as we grow in understanding. It is my hope that people make decisions based on reality rather than myth. That's all I'm going for and I apologize if I came across wrong.

 

[BuckeyeChuck]

I found nothing wrong with your explanation or that of your friend. We should be thankful that engineers spend multiple courses in college learning how to understand, calculate, and harness forces; without this detailed understanding, the machines in our lives would be less effective, less safe, and more expensive. We should also be happy that for some engineers, their learning merely starts in college and that they see situations they encounter in real lfe as a chance to enhance their knowledge.

Regarding the existence and nature of centripetal and centrifugal force, frame of reference is very important. When you're in a turning car, there are clearly TWO forces at work: the force the turning car applies to you, and the force you apply to the turning car. They are equal and opposite, which is why there is no relative motion between you and the car. These forces are not imaginary so the math must support their existence, and it does. Frame of reference helps us understand how the math applies to this reality.

Oh, I forgot to say... my Shield 9 does not have the issue referenced by this safety alert. The gun was manufactured in 2013.

 

[motomed]

so why don't revolvers have fancy triggers?

Still not buying the deceleration explanation. The only way the trigger is pulled is if it doesn't decelerate that fast, as in it keeps moving against the suddenly stopped frame. The only way it generates the astronomical G's you guys are reporting is if it comes to a sudden stop right along with the frame..... in which case there's still not a problem because the trigger didn't pull.

and given the calculations above, wouldn't the bottom half of the trigger also be subject to the same uber G force and just rotate the safety notch right out of the way?

also, why don't revolvers have fancy triggers?

 

[motomed]

and what did kahr say when you called and told them how stupid they are? haven't gotten the recall notice on my p380 yet...?

 

[motomed]

just read back through it again.... your 1500g calculation has the trigger with a final velocity of a big fat zero to get to that conclusion.... If the trigger is moving at a velocity of nothing, we're good here.....

 

[Rastoff]

just read back through it again.... your 1500g calculation has the trigger with a final velocity of a big fat zero to get to that conclusion.... If the trigger is moving at a velocity of nothing, we're good here.....

Look, it's not the final velocity that makes it so, it's the change in velocity over a short period of time and distance. You can't look at one number and come up with the answer. You have to look at is as a whole.

 

[motomed]

Look, it's not the final velocity that makes it so, it's the change in velocity over a short period of time and distance. You can't look at one number and come up with the answer. You have to look at is as a whole.

correct, but the acceleration calculations you posted with an astronomical value assume an instantaneous change and a trigger pull of 1 mm, which is not reality. the bottom line is something so light that would have to decelerate over a distance and time that is not zero would never produce enough force to overcome the trigger pull weight.

still curious why revolvers don't have these things.

 

[Rastoff]

I'm truly sorry that you don't understand this motomed. I'll try again.

The 1mm is for the frame. It's an assumed amout that the frame will distort as it impacts a hard surface. Obviously the trigger would continue to move, once the frame stops, if it didn't have the trigger safety.

I would explain the revolver thing to you, but it's clear you're not here to learn.

 

[RobzGuns]

It's an assumed amount that the frame will distort as it impacts a hard surface. Obviously the trigger would continue to move, once the frame stops, if it didn't have the trigger safety.

Seems like people no longer question the job of seat belts, but they do still question the function of a trigger safety in Striker Fire pistols. (I know... a VERY loose example of how they both relate to Newton's 3rd Law of Motion)
While trigger safeties vary from Brand to Brand, Glock, XD, S&W, Ruger's Striker Fire pistols... pretty much everyone who makes Striker Fire pistols uses 'em. If they didn't serve a purpose, I doubt if any of them would bother to incorporate them into the systems. Are they ALL wrong and simply messing with us??
Just my thoughts on it.

 

[motomed]

I'm truly sorry that you don't understand this motomed. I'll try again.

The 1mm is for the frame. It's an assumed amout that the frame will distort as it impacts a hard surface. Obviously the trigger would continue to move, once the frame stops, if it didn't have the trigger safety.

I would explain the revolver thing to you, but it's clear you're not here to learn.

keep trying.... 1mm in the calculations you provided represent the total distance over which the deceleration occurs, and this is not accurate for our purposes.... we are interested in that distance PLUS, more importantly, the distance over which the deceleration of the trigger occurs.... aka.... the length of the trigger pull before it breaks....

 

[motomed]

Seems like people no longer question the job of seat belts, but they do still question the function of a trigger safety in Striker Fire pistols. (I know... a VERY loose example of how they both relate to Newton's 3rd Law of Motion)
While trigger safeties vary from Brand to Brand, Glock, XD, S&W, Ruger's Striker Fire pistols... pretty much everyone who makes Striker Fire pistols uses 'em. If they didn't serve a purpose, I doubt if any of them would bother to incorporate them into the systems. Are they ALL wrong and simply messing with us??
Just my thoughts on it.

if the inertia of the trigger is what the trigger safety is designed to protect against, the type of action would be irrelevant. Why don't revolver triggers have this problem with trigger inertia when dropped?

 

[S&WForty]

My disclaimer is that I've never disassembled a pistol beyond what is required for cleaning per the manual.

Early calculations by someone assumed 100 g for the "trigger and trigger bar assembly." Do they move in the same direction? When we look at the effects of inertia on the "trigger," should it be only the piece of plastic the finger pulls or that and the trigger bar?

If it is only that piece of plastic, what is its mass? A few grams? If so, the huge acceleration values don't translate into huge forces.

The base units of F=ma are Newtons, kilograms, and m/s^2. If you crunch the numbers on the plastic trigger where it has a mass of 1-5 grams, that is 0.001-0.005 kg.

The ~14,500 m/s^2 acceleration (~1,500 g's) from the conservation of energy equation looked fine to me. The post that explained it took me down memory lane! Anyway...

F=m x a
F= 0.005 g x 14,500 m/s^2 = 72.5 N.
(1 lb = 4.448 N)

Force due to inertia on a 5 gram trigger is 72.5 N or 16.3 lbs. The trigger may have less mass than 5 grams.

The statement elsewhere about forces being vectors was an excellent point. The direction must line up exactly for 100% of the force to be realized.

Don't forget my disclaimer above! The trigger bar mass may need to be included, but I suspect vectors are required if you must take it into effect.

As I've stated before, I know S&W did not design this safety for no reason, but I don't think it's as likely to be needed as the huge (though correct) number of "1,500 g's" implies.

There is something else that could be behind the need of the safety: "safety factors." By applying an engineering SF of 2.0 (or whatever) in their design, S&W may have then said, "Yeah, we need to include an inertia safety on the trigger."

Though there is a lot of disagreement here, there is some good thinking coming from all sides. Motomed makes a good point. The acceleration of 1,500 g's is or is nearly instantaneous. The trigger is not going to reach the break over point instantly.

Many thanks to Rastoff's friend's input, too. And the poster that mentioned in vectors.

 

[RobzGuns]

if the inertia of the trigger is what the trigger safety is designed to protect against, the type of action would be irrelevant. Why don't revolver triggers have this problem with trigger inertia when dropped?

Not quite... The action of a true DAO revolver trigger and the action of Striker Fire pistol are apples and oranges (or Apples and PCs).
While some manufacturers advertise their Striker Fire system as being DAO, they are all much closer to being a SA trigger than that of a revolver.

Compare these two visual examples. If a picture is worth 1000 words, then the 2 links below speak volumes:
Glock Pistol Animation
Modern Revolver Animation
Unfortunately, the Glock 'Safe Action' illustration is the only Striker Fire example they show, but it's pretty close to the M&P system.

 

[motomed]

Not quite... The action of a true DAO revolver trigger and the action of Striker Fire pistol are apples and oranges (or Apples and PCs).
While some manufacturers advertise their Striker Fire system as being DAO, they are all much closer to being a SA trigger than that of a revolver.

Compare these two visual examples. If a picture is worth 1000 words, then the 2 links below speak volumes:
Glock Pistol Animation
Modern Revolver Animation
Unfortunately, the Glock 'Safe Action' illustration is the only Striker Fire example they show, but it's pretty close to the M&P system.

Inertia Trigger is not concerned with any of this, and is only concerned with how much force it needs to generate to go bang. a revolver with a 6lb trigger pull of x length would be just as susceptible to inertia drop trigger as a striker fire pistol with a 6lb trigger pull of x length. what mechanisms generate that force or resistance is irrelevant to Inertia Trigger.

 

[RussC]

if the inertia of the trigger is what the trigger safety is designed to protect against, the type of action would be irrelevant. Why don't revolver triggers have this problem with trigger inertia when dropped?

My 12 year old daughter can't pull back a double action trigger on a revolver. You could drop it from the moon and that 100 lb trigger isn't going anywhere upon impact.

Russ

 

[jframe]

This thread has deteriorated from a gun related issue into one dealing with physics. Thought I might find new info on the safety alert, but it just ended up making my head hurt with facts and figures. Hell, it's hard enough for this boy to make change, much less understand all those theorems.

 

[MrJT]

I'm glad they described the issue well and explained how to test your trigger. I tested mine as soon as I read the announcement on their website.

 

[RobzGuns]

The only gripe I have about the Safety Alert, is that they didn't email those who registered their pistols and supplied them with an email address. Since most of us don't visit the S&W site daily, we basically had to find out via the Forums.

Other than that... Very well presented description of the problem and how to test ours. Pretty Good Job, over-all.

 

[Soldiernurse]

My Shield trigger is working properly, per SA Website directions.

However, my XDs45 S# is on the recall roster.